[dm-devel] [PATCH 2/2][v2] blk-plug: don't flush nested plug lists

[Jeff Moyer]

Dave Chinner <david at fromorbit.com> writes:

> On Tue, Apr 07, 2015 at 02:55:13PM -0400, Jeff Moyer wrote:
>> The way the on-stack plugging currently works, each nesting level
>> flushes its own list of I/Os.  This can be less than optimal (read
>> awful) for certain workloads.  For example, consider an application
>> that issues asynchronous O_DIRECT I/Os.  It can send down a bunch of
>> I/Os together in a single io_submit call, only to have each of them
>> dispatched individually down in the bowels of the dirct I/O code.
>> The reason is that there are blk_plug-s instantiated both at the upper
>> call site in do_io_submit and down in do_direct_IO.  The latter will
>> submit as little as 1 I/O at a time (if you have a small enough I/O
>> size) instead of performing the batching that the plugging
>> infrastructure is supposed to provide.
>
> I'm wondering what impact this will have on filesystem metadata IO
> that needs to be issued immediately. e.g. we are doing writeback, so
> there is a high level plug in place and we need to page in btree
> blocks to do extent allocation. We do readahead at this point,
> but it looks like this change will prevent the readahead from being
> issued by the unplug in xfs_buf_iosubmit().

I'm not ignoring you, Dave, I'm just doing some more investigation and
testing.  It's taking longer than I had hoped.

-Jeff